1. Field of the Invention
The present invention relates to a novel method for separating and purifying carboxylic acids, particularly long chain dicarboxylic acids, from fermentation broths, and to improved methods for producing carboxylic acids by fermentation.
2. Background of the Related Prior Art
Carboxylic acids, particularly long chain dicarboxylic acids are commercially important products utilized in the production of polymers, adhesives, perfumes, and antibiotics. As is known, production of dicarboxylic acids on an industrial scale typically occurs by fermentation with various microorganisms, e.g., yeast, using alkanes or fatty acids as the carbon source. The broth obtained from fermentation generally contains a significant amount of impurities such as the biomass from spent microorganisms, proteins, amino acids, fatty acids, sugars, carbohydrates, etc.
Several techniques exist for separating carboxylic acids from the various impurities present in the fermentation broth. For example, U.S. Pat. No. 4,075,093, describes a process of separating citric acid and isotropic acid from fermentation broths using membranes of selective permeability composed of polysulfones modified by acid groups, or of vinyl polymers which are chemically modified by acid or basic groups, within a particular pressure range.
U.S. Pat. No. 4,904,389 describes a process for separating saturated dicarboxylic acids from their mono and di-unsaturated analogs present in a fermentation broth using a membrane filtration system at a pressure of up to 20 bar and adjusting the pH of the fermentation broth to a pH of from 4 to 11, such that the membrane is selectively impermeable to at least one of the dicarboxylic acids present in the fermentation broth.
While the aforementioned membrane filtration methods may be useful in separating carboxylic acids from fermentation broths, such methods are time-consuming and require the prior separation of microorganism cells from the aqueous portion of the fermentation broth using various techniques such as filtration, centrifugation, etc.
Carboxylic acids may also be separated from other impurities in the fermentation broth by techniques involving separation of the biomass from the fermentation broth, precipitation of the carboxylic acid from the fermentation broth, and recovery of the crystals from the broth. For example, U.S. Pat. No. 5,612,131 describes a process for recovering organic acids, particularly citric acid, from fermentation broth by removing the biomass of spent microorganism cells and treating the cell-free broth with a strong acid or salt to precipitate out the organic acid, followed by separating the crystals to recover the organic acid.
U.S. Pat. No. 4,339,536 describes the separation of long chain dicarboxylic acids from a fermentation broth by adding diatomaceous earth to the fermentation broth, filtering the broth under pressure, and then precipitating the dicarboxylic acid from the broth using a mineral acid and heating.
JP No. 176498 describes the separation of dicarboxylic acids from fermentation broth by removing the biomass from the broth, heating and treating the cell-free broth with activated carbon, regulating the pH of the broth to below 4.0 using an inorganic acid, and recovering the crystallized dicarboxylic acid from the broth.
JP No. 086443 describes the refining of dicarboxylic acids from fermentation broth, which includes separating the biomass from the fermentation broth, heating the cell-free broth at a pH above 7.0, regulating the pH to below 3.0 using an inorganic acid, and recovering the dicarboxylic acid crystals.
The aforementioned processes requiring precipitation of the carboxylic acid from the fermentation broth are inefficient in that the precipitation process itself is time-consuming and requires the prior separation of the spent microorganism cells from the broth. Further, the crystals obtained from the precipitation process yield a less purified product than is desired.
Accordingly there continues to be a need for improved methods to efficiently separate carboxylic acids, particularly dicarboxylic acids, from a fermentation broth on an industrial scale to yield a substantially pure product at high yields and at reasonably low operating costs.